Children with acute immobility (whether by neurological or chemical paralysis, or for conditions related to medical or surgical status) are at increased risk for the development of deep venous thromboses (DVTs). Certain variables may increase this risk including history of previous DVT, hypercoagulability disorder, or trauma. Instrumentation such as central or femoral venous catheterization can increase this risk further. Pulmonary embolus has also been reported in children who have developed DVTs, so there is the potential for significant morbidity. Post phlebiotic syndrome is also a potential cause for morbidity, though it has been reported only occasionally in children. Since there are prophylactic measures for DVTs, an estimation of the prevalence is important to compare the costs and benefits of prophylactic measures and medications. Very few studies have been performed to document DVTs in children, yet most medical practitioners working with these groups of children have had clinical experience with DVTs in the pediatric age group. The purpose of this study is to determine the prevalence of deep venous thromboses in children who are admitted to the UW Children's Hospital Pediatric Intensive Care Unit and the hospital's Burn Unit, and who are either paralyzed (neurological or chemical paralysis); immobilized because of casting or traction devices; or who are at bed rest for medical or surgical conditions. The long term goal of this research project is formulate a future study to examine the efficacy of DVT prophylaxis in this patient population.